Modern computing devices may emit radio frequencies anywhere in the range 30 Mhz to several Ghz, while compliance with national regulations often requires that products only emit within certain strict limits in the latter frequency range to prevent interference with communications. The devices need therefore be tested in environments provided by anechoic chambers the demand for which consequently regularly grows in the industry.
As stated in Scientific Report No. 105 from the Department of Electrical and Computer Engineering, University of Boulder, Colorado, USA, present-generation anechoic chambers exhibit excellent broad-band suppression of reflected waves in the microwave region using pyramidal absorbers. The very low reflections from pyramidal absorbers result from the fact that incident microwaves reflect several times from the cones before finally being reflected back into free space; since a fraction of the incident wave is absorbed at each bounce, the microwaves are very much diminished by the time they reflect back from an array of absorbers. The same type of absorber is sometimes used for lower frequency (30Mhz) waves. At low enough frequencies, however, the waves become much longer than the spacing between adjacent absorbers. Their skin-depths in the absorbing materials likewise become long compared to the size of the pyramids. This makes pyramid absorbers of limited usefulness for anechoic chambers to be used at lower frequencies.
At these lower frequencies, it is also possible to achieve low reflection coefficients using a single-layer dispersive absorber. Still another approach to designing anechoic chambers has been the use of multilayer absorbers. These absorbers are built to minimize reflection in a specified range of frequencies. Design of multilayer absorbers has been successfully performed using cut-and-try methods, Smith-chart methods and by numerical optimization techniques.
Finally, as stated again in the above-mentioned Scientific Report, it has been considered to combine the advantages of multilayer absorbers with those of pyramid absorbers. This is accomplished by replacing the top layer of a multilayer structure with a layer of pyramid absorbers. In such a structure, the effective material properties match continuously to the external medium, so good performance is expected in the range ot frequencies between the design frequency and the microwave region, where quasi-optical techniques are applicable. Such a structure is shown in FIG. 1. The advantage of this approach is that the higher frequency waves do not penetrate into the backing behind the pyramids due to their short wavelengths and skin depths, so microwave performance should be equal to that of absorbers originally designed for microwaves. The remaining layers can be adjusted so as to minimize reflection for lower frequencies.
The design optimization of an absorber comprising a twisted pyramid combined with a multilayer structure therefore proceeds in two phases:
computation of the reflection and transmission properties (S parameters) of the absorber, and PA0 a search for the design which minimizes the overall reflection.
Since ordinary absorbing pyramids are two-dimensionally periodic, it is feasible to compute their averaged or "effective" permittivity and permeability accurately with respect to fields which vary slowly with distance compared to the pyramids themselves. The technique for doing this is known as homogenization. Once the averaged material properties are computed, they may be looked up as needed, and used to solve for the S-parameters of the array of pyramids. Reflection from the overall structure can then be easily calculated from the S-parameters and the known properties of the backing layers. The method of computation is summarized below as extracted from the above-mentioned Scientific Report, and referring back to it. The backing layers are considered free to vary within certain practical bounds. The size and composition of these layers are controlled by a set of variables and constraints which constitute a problem space which is a subspace of Rn, where n is the total number of variables used to specify the backing layers. Then the design optimization process itself can start.